1. Field of the Invention
The present invention relates to an electrophoretic display liquid, and more particularly to an electrophoretic display liquid including particles, which reversibly changes its optical properties by applying an electric field thereto. In addition, the present invention relates to an electrophoretic display medium, method and device using the electrophoretic display liquid.
2. Discussion of the Background
An electrophoretic display medium having a closed cell in which two substrates, at least one of which is transparent, face each other with one or more spacers therebetween and which includes a display liquid including a dispersion medium and color particles dispersed in the dispersion medium whose color is different from the color of the color particles is known. In the electrophoretic display device, images are formed on the transparent substrate by applying an imagewise electric field to the display medium.
As the dispersion medium, xylene, isoparaffin and the like liquid are known. As the colored particles, titanium dioxide and the like pigment particles are known. In addition, additives such as a dye which is used for increasing the contrast of a formed image, a dispersant (e.g., a surfactant) which is used for dispersing the particles in the dispersion medium, and a charge imparting agent which imparts a charge to the color particles, are added thereto if desired. When an electric field is applied to the cell including the display liquid, the color particles move toward the transparent substrate of the cell, and thereby the transparent substrate achieves the color of the color particles. When an opposite electric field is applied thereto, the colored particles move toward the substrate opposite to the transparent substrate, and thereby the transparent substrate achieves the color of the dispersion medium which is colored by a dye.
Recently such an electrophoretic display device attracts considerable attention because of having the following advantages:
(1) the display device can display an image by controlling only the direction of the electric field applied thereto;
(2) the display liquid can be prepared by materials which can be easily obtained and which have relatively low costs;
(3) the images formed in the display device have a field of view as wide as general printed documents (i.e., the image can be easily observed even at a point such that an angle formed by the point and the normal line of the display plane is large);
(4) the display device consumes relatively low electric power; and
(5) the image formed in the display device can be used as a memory (i.e., the image can be read even when application of an electric field to the display medium is stopped).
An electrophoretic display device in which microcapsules including a colored dispersion medium and color particles having a different color and dispersed in the dispersion medium are sandwiched by electrodes is disclosed in Japanese Laid-Open Patent Publication No. 1-86116 (i.e., Japanese Patent No. 2551783). Such a display device has advantages in that the device is simple, and uneven images due to maldistribution of the particles can be avoided. However, the device has a drawback in that contrast of images is poor because of using a colored dye solution and pigment particles which are dispersed in the colored dye solution.
As mentioned above, such an electrophoretic display device includes a display liquid typically including particles such as inorganic pigments having a high refractive index (e.g., titanium dioxide) dispersed in a colored dispersion medium which includes a dye. In such a display device, the formed images have poor contrast because the color of the pigment is displayed as a mixed color of the pigment and the dispersion medium including a dye. The reason of this problem is considered to be that the dye adsorbs the pigment and the dispersion medium including the dye penetrates between the pigment particles. Therefore, this problem cannot be avoided in such a display device. This problem prominently occurs in a display device using a white pigment. The display device therefore produces images whose background area has poor whiteness. This is a fatal drawback in such display media.
In attempting to solve this problem, the following methods have been disclosed:
(1) a dye which colors the dispersion medium but does not adsorb the pigment dispersed in a display liquid is used (described by Philips Lab. in Conference Record of 1980 Biennial Disp. Res. Conf.);
(2) a dispersion medium including a dye at a relatively low concentration is used (described by Xerox PaloAlto in Proc. SID, Vol. 18, 3/4, 1977); and
(3) the dye concentration, and pigment concentration are optimized while a suitable surfactant is used (described by Matsushita Electric Works, Ltd. in Proc. SID, Vol. 18, No. 3/4, 1977).
However, these methods induce other problems in that the density of images decreases and response of the device deteriorates as well as it is hardly effective for improving image contrast.
In attempting to solve the above-mentioned problems of the display device including a dispersion medium including a dye, display devices which do not use a dye solution have been proposed. For example, Japanese Laid-Open Patent Publication No. 62-269124 discloses an electrophoretic display device having a closed cell which has two electrodes, at least one of which is transparent and which are opposed each other with a spacer therebetween. In the closed cell, at least two kinds of electrophoretic particles whose color and electrophoretic properties are different form each other are dispersed in a colorless liquid having a high insulation property and a low viscosity. Since these two kinds of color particles have charges having opposite polarities (i.e., one has a positive polarity and the other has a negative polarity), the particles tend to aggregate due to electric attraction. Therefore, a problem which occurs is that the resultant image has poor contrast because the image has a mixed color thereof.
In addition, WO98/03896 discloses a display device including a display liquid which is the same as that disclosed in Japanese Laid-Open Patent Publication No. 62-269124 and which is microencapsulated. However, the display device has the same drawback as that the image has poor contrast because the image has a mixed color of the two kinds of color particles.
In addition, Japanese Laid-Open Patent Publication No. 63-50886 discloses an electrophoretic display device having a closed cell which has two electrodes, at least one of which is transparent and which are opposed each other with a spacer therebetween. In the closed cell, at least two kinds of electrophoretic particles whose color and electrophoretic speed are different form each other are dispersed in a colorless liquid having a high insulation property and a low viscosity. Since these two kinds of color particles have charges having the same polarity but have different electrophoretic speed, it is difficult to control the color of images particularly when images are repeatedly formed. This is because it is hard to control exchanging of one kind of particles, which are present on the substrate of the cell as an image, with another kind of particles contained in the cell when the image displayed is changed. Therefore, such a display device also has a drawback in that images having good contrast cannot be obtained. In addition, it is difficult for the display device to display different colors at the same time and therefore it has poor practicality.
In attempting to solve the problem (i.e., formation of aggregates of different color particles) of the above-mentioned display device including two or more kinds of particles having different charge polarities, methods in which a charge controlling agent is added in the display liquid, or particles, which are subjected to a surface treatment so that the particles repulse the other kind of particles using steric repulsion effect, are used in the display liquid (Japanese Patent Publication Ko-hyo No. 8-510790). However, it is difficult to perfectly avoid aggregation of particles by such methods. Therefore, images having good contrast cannot be obtained by such methods.
In addition, Japanese Laid-Open Patent Publication No. 10-149117 discloses an ink composition including white particles which includes a resin and a white pigment and which is used for concealment purpose, colored particles and a solvent. The white particles are a complex of the resin and the white pigment and prepared by kneading a resin and a white pigment and then pulverizing the mixture. Alternatively, the particles are prepared by polymerizing a dispersion in which a monomer and a white pigment are mixed and then making aggregates of the polymerized particles. Namely, the complex white particles are almost the same as the white particles used as a colored pigment in the above-mentioned electrophoretic display device using a dyed dispersion medium and the colored pigment. Accordingly, the complex white particles also forms aggregates with the other pigment having a different color which is used for displaying images (for example, a magnetic powder, or a mixture color pigment thereof) because the white particles do not have a function reducing aggregates. Therefore, the problem in that images having poor contrast are produced has not yet be solved.
Because of these reasons, a need exists for an electrophoretic display device which can produce images having good contrast.
Accordingly, an object of the present invention is to provide an electrophoretic display liquid which can display images having high contrast.
Another object of the present invention is to provide an electrophoretic display medium which can display images having high contrast.
Yet another object of the present invention is to provide an electrophoretic display device which can display images having high contrast.
A further object of the present invention is to provide an electrophoretic displaying method, which can display images having high contrast.
Briefly these objects and other objects of the present invention as hereinafter will become more readily apparent can be attained by an electrophoretic display liquid in which a first granular material having one or more hollows therein (hereinafter referred to as hollow particles) and a second granular material (hereinafter referred to as pigment particles) whose color is different from the color of the first granular material are dispersed in a dispersion medium.
The hollow particles preferably include organic polymer particles having a hollow therein. In addition, the surface of the pigment particles is preferably treated with a coupling agent. The coupling agent preferably includes a titanate coupling agent.
The electrophoretic display liquid may be microencapsulated.
In another aspect of the present invention, an electrophoretic display medium is provided which includes a closed cell in which a pair of substrates are configured to face each other with one or more spacer therebetween and in which the above-mentioned display liquid is contained, wherein at least one of the pair of substrates includes an electrode at an inside surface thereof and at least one of the pair of substrates is transparent.
The cell may be divided into a plurality of cells with a matrix material. For example, the display liquid is dispersed in a matrix material such as a resin.
In addition, the display medium may include an information recording portion other than the electrophoretic display portion.
In yet another aspect of the present invention, an electrophoretic display device including the display medium, and an image writing device which can be attached with and detached from the display medium and which includes an electric field forming device applying an electric field to the display medium according to image signals to form images in the display medium. The electric field forming device is preferably an electrode array, an ion gun array, or a device having a plurality of signal electrodes, a plurality of scanning electrodes, and switching elements which are provided at the intersections of the signal electrodes and scanning electrodes.
In a further aspect of the present invention, an electrophoretic displaying method is provided which includes providing the electrophoretic display medium mentioned above, and applying an electric field in the display medium to form an image in the display medium by electrophoresis.
These and other objects, features and advantages of the present invention will become apparent upon consideration of the following description of the preferred embodiments of the present invention taken in conjunction with the accompanying drawings.